1. Field of the Invention
The present invention relates generally to the fabrication of semiconductor chip packages, and more particularly, to a semiconductor package structure having a constraint stiffener for efficient cleaning and underfilling of a chip mounted on a substrate.
2. Brief Discussion of the Related Art
In the microelectronics industry, a chip carrying an integrated circuit is commonly mounted on a package carrier, such as a substrate, a circuit board or a leadframe, that provides electrical connections from the chip to the exterior of the package. In one such packaging arrangement called flip chip mounting, the chip includes an area array of electrically conductive contacts, known as bond pads, that are electrically connected to corresponding area array of electrically-conductive contacts on the substrate, known as solder balls or bumps. Typically, the solder bumps are registered with the bond pads and a reflow process is applied to create electrical connections in the form of solder joints between the chip and the substrate. The process of flip chip mounting results in a space or gap between the chip and the substrate.
The chip and the substrate are usually formed of different materials having mismatched coefficients of thermal expansion. As a result, the chip and the substrate experience significantly different dimension changes when heated that creates significant thermally-induced stresses in the electrical connections between the chip and the substrate. If uncompensated, the disparity in thermal expansion can result in degradation in the performance of the chip, damage to the solder joints, or package failure. As the size of the chip increases, the effect of a mismatch in the coefficient of thermal expansion between the chip and the substrate becomes more pronounced. In stacked die packages, the mismatch in coefficient of thermal expansion between the die laminate and the package may be even greater than in single die packages. The failure mechanism in stacked die packages may shift from solder joint damage to die damage.
To improve the reliability of electrical connections in flip chip package assemblies, it is common in the microelectronics industry to fill the gap between the chip and the substrate with an encapsulant material, or underfill. The underfill increases the fatigue life of the package and improves the reliability of the electrical connections by reducing the stress experienced by the electrical connections during thermal cycling or when the chip and the substrate have a significant temperature differential. The underfill also isolates the electrical connections from exposure to the ambient environment by hermetically sealing the gap and lends mechanical strength to the package assembly for resisting mechanical shock and bending. The underfill further provides a conductive path that removes heat from the chip and that operates to reduce any temperature differential between the chip and the substrate. As a result, the underfill significantly increases the lifetime of the assembled package.
To further enhance the rigidity of the package assembly, stiffeners are typically employed in the package assembly. Due to the highly rigid material of the stiffener, the package assembly would be less likely to be subject to package warpage.
In the conventional package assembly, prior to dispensing the underfill in the gap between the chip and the substrate, a cleaning solvent is injected to the gap to clean away remnant solder flux. However, a drawback is that the cleaning solvent would be partly obstructed by the stiffener, making the cleaning process very difficult to carry out efficiently and thoroughly. When some solder flux is still left, it would cause the subsequently formed flip chip underfill layer to have voids, which would considerably degrade the quality and reliability of the finished package product.
Another drawback to the conventional package assembly is that during the time the underfill is injected in the gap between the chip and the substrate, the underfill dispenser will be partly obstructed by the stiffener, making the underfill dispensing process a time consuming task and not easy to carry out thoroughly.
Accordingly, what is needed in the art is a semiconductor package structure having an improved stiffener design that overcomes the problems discussed above.